1. Field of the Invention
The present invention relates to a variable-shape mirror that permits the shape of a mirror surface thereof to be varied, and more particularly relates to prevention of the deformation that occurs when the mirror surface of a variable-shape mirror is varied. The present invention also relates to an optical pickup device provided with a variable-shape mirror of which the mirror surface is prevented from deformation.
2. Description of Related Art
When information is read from or written to an optical disc such as a CD (compact disc) or DVD (digital versatile disc) by the use of an optical pickup device, the relationship between the optical axis of the optical pickup device and the disc surface should ideally be perpendicular. In reality, however, when the disc is rotating, their relationship does not always remain perpendicular. As a result, with an optical disc such as a CD or DVD, when its disc surface becomes inclined relative to the optical axis, the optical path of laser light is so bent as to produce coma aberration.
When coma aberration is produced, the spot of laser light shone on the optical disc deviates from the proper position, and, when the coma aberration becomes larger than permitted, inconveniently, it becomes impossible to accurately write or read information. For this reason, there have conventionally been proposed methods for correcting coma aberration as described above and other aberrations by the use of a variable-shape mirror.
For example, JP-A-H5-333274 proposes a method of performing phase control by varying the shape of the mirror itself of a variable-shape mirror by the use of a plurality of actuators. Disadvantageously, however, this method is unsuitable for use in a small component such as an optical pickup device because it does not take wiring and other factors into consideration. Moreover, it is difficult, both technically and in terms of cost, to miniaturize multi-layered piezoelectric elements used as those actuators.
JP-A-2003-172811 proposes a variable-shape mirror that has, between a mirror portion and a mirror support portion supporting the mirror, a sealed space that is filled with a fluid material such as a liquid, gaseous, or gel material. According to the discloser, by this method, it is possible to properly control the flatness of the initial shape of the mirror surface of the mirror portion. Disadvantageously, however, it is technically rather difficult to provide a sealed space in a variable-shape mirror. Moreover, even though the flatness in the initial state is maintained, the mirror surface may deform when the shape of the mirror is varied. If the mirror surface deforms, it is impossible to correct coma aberration.
JP-A-2004-109562 proposes a method, for use in a wavefront aberration correction mirror that corrects aberrations by displacing the mirror surface of a mirror base by the use of a piezoelectric element, whereby, for the purpose of reducing the deformation of the mirror due to the presence of adhesive, the adhesive layer between the mirror base and the piezoelectric element is formed thin. Disadvantageously, however, by this method, certainly the adhesive layer can be formed thin, but, since the mirror and the piezoelectric element are bonded together with adhesive, when the shape of the mirror is varied with the piezoelectric element, the mirror surface may locally deform. When deformation due to the mirror and the piezoelectric element being bonded together occurs, as described above, it is impossible to properly correct coma aberration.
Incidentally, a variable-shape mirror is required to be designed to offer as large a mirror displacement as possible, because, with an insufficient mirror displacement, it is impossible to satisfactorily correct aberrations.